Making a diagnosis of ankylosing spondylitis in patients with chronic back pain can be difficult at an early stage—that is, before radiographic sacroiliitis is definitely present (also referred to as axial spondyloarthritis (SpA) at the preradiographic state). We recently proposed to diagnose patients at this early stage by probability estimations¹ based on a pretest probability (p_pre) of 5% in patients with chronic back pain.² To facilitate the probability calculation in each patient, we subsequently³ proposed the use of likelihood ratios (LR).⁴ We suggested that the diagnosis could be considered definite if the post-test probability (p_post) is ⩾90% (LR product ⩾171), probable if the post-test probability is 80–90% (LR product …